Conventional light sources, such as incandescent, halogen and fluorescent lamps, have not been significantly improved in the past twenty years. However, light emitting diode (“LEDs”) have been improved to a point with respect to operating efficiency where LEDs are now replacing the conventional light sources in traditional monochrome lighting applications, such as traffic signal lights and automotive taillights. This is due in part to the fact that LEDs have many advantages over conventional light sources. These advantages include longer operating life, lower power consumption, and smaller size.
LEDs are typically monochromatic semiconductor light sources, and are currently available in various colors from UV-blue to green, yellow and red. Due to the narrow-band emission characteristics, monochromatic LEDs cannot be directly used for “white” light applications. Rather, the output light of a monochromatic LED must be mixed with other light of one or more different wavelengths to produce white composite light. Some conventional LEDs include a wavelength-conversion mechanism to convert some of the original light generated by the LEDs into longer different wavelength light, which combines with the original light to produce white composite light. One such type of LEDs of interest uses an LED die with a fluorescent substrate as the wavelength-conversion mechanism to convert the light emitted from an active layer of the LED die into longer wavelength light. As an example, the active layer of the LED die may be Gallium Indium Nitride (GaInN) active layer that emits light having a peak wavelength of approximately 430 nm, and the fluorescent substrate of the LED die may be a Gallium Nitride (GaN) substrate doped with oxygen, carbon or nitrogen vacancy that converts some of the light emitted from the GaN active layer into light having a peak wavelength of approximately 550 nm.
A concern with the conventional LEDs that use a fluorescent substrate as a wavelength-conversion mechanism is that the resulting white composite light from the LEDs may be deficient in certain wavelengths, e.g., in the red wavelength range of the visible spectrum (˜620 nm to ˜800 nm). As such, these LEDs do not have high color rendering characteristics.
Another concern is that since the original light and the converted light originate from different locations within an LED die, the original light and the converted light may not blend together well, resulting in a composite light that has a non-homogeneous color. As an example, the composite output light from the central portion of the LED may appear bluer than the light from the perimeter of the LED.
In view of these concerns, there is a need for an LED and method for emitting white composite light with high color rendering characteristics and more homogeneous color.